btrfs: zoned: introduce a minimal zone size 4M and reject mount

Zoned devices are expected to have zone sizes in the range of 1-2GB for
ZNS SSDs and SMR HDDs have zone sizes of 256MB, so there is no need to
allow arbitrarily small zone sizes on btrfs.

But for testing purposes with emulated devices it is sometimes desirable
to create devices with as small as 4MB zone size to uncover errors.

So use 4MB as the smallest possible zone size and reject mounts of devices
with a smaller zone size.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/zoned.c

@@ -51,11 +51,13 @@
 #define BTRFS_MIN_ACTIVE_ZONES		(BTRFS_SUPER_MIRROR_MAX + 5)
 
 /*
- * Maximum supported zone size. Currently, SMR disks have a zone size of
- * 256MiB, and we are expecting ZNS drives to be in the 1-4GiB range. We do not
- * expect the zone size to become larger than 8GiB in the near future.
+ * Minimum / maximum supported zone size. Currently, SMR disks have a zone
+ * size of 256MiB, and we are expecting ZNS drives to be in the 1-4GiB range.
+ * We do not expect the zone size to become larger than 8GiB or smaller than
+ * 4MiB in the near future.
  */
 #define BTRFS_MAX_ZONE_SIZE		SZ_8G
+#define BTRFS_MIN_ZONE_SIZE		SZ_4M
 
 #define SUPER_INFO_SECTORS	((u64)BTRFS_SUPER_INFO_SIZE >> SECTOR_SHIFT)
 
@@ -402,6 +404,13 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
 				 zone_info->zone_size, BTRFS_MAX_ZONE_SIZE);
 		ret = -EINVAL;
 		goto out;
+	} else if (zone_info->zone_size < BTRFS_MIN_ZONE_SIZE) {
+		btrfs_err_in_rcu(fs_info,
+		"zoned: %s: zone size %llu smaller than supported minimum %u",
+				 rcu_str_deref(device->name),
+				 zone_info->zone_size, BTRFS_MIN_ZONE_SIZE);
+		ret = -EINVAL;
+		goto out;
 	}
 
 	nr_sectors = bdev_nr_sectors(bdev);